Investment composition



March 9, 1943. R. Nl-:IMAN ETAL 2,313,086

INVESTMENT COMPOSITION Filed sept. 5, 193e 4o Pneu-NT PoTAssluM Ion/DE ROBERT NEIMAN RoERT C. ERNST 3y nMuNo A. STElNeocn Q. WM ATTORNEY Patented' Mar. 9, 1943 INVESTMENT COMPOSITION Robert Neiman, Robert C. Ernst, and Edmund A. Steinbock, Louisville, Ky.; said Neiman and said Ernst assignors to University of Louisville, Louisville, Ky., a corporation of Kentucky Application September 5, 1939, Serial No. 293,449l

12 Claims.

This invention relates to investment compositions for making refractory molds into which metals, such as precious metals or other suitable casting material, can be cast into objects which must conform with great exactness to a predetermined size and shape.

This invention will be described in its application to making dental castings such as inlays, which castings must be very accurately made as far as size and shape are concerned, but it is to be understood that the composition has general appli-l cation in industry generally.

In making dental inlays or inlay restorations, wax models of the exact size and shape of the cavity to be filled are first made and then invested in a composition generally known as investment compositions, that is, a plastic .mass which sets to a hard refractory mold. After the investment sets, the mold is heated to a temperature of 800 F. to 1800 F., in order to melt and burn out the wax and also to prepare the mold for casting. Any suitable or desirable method may then be employed in casting, usually, use is made of some machine which forces the molten metal, under pressure, into the cavity formerly occupied by the wax. The mold is then cooled by plunging it into water and is then broken apart to obtain the casting. The casting is then cleaned and polished for use. Obviously, if the mold used had expanded sufficiently, prior to the casting process, to counteract exactly the casting shrinkage of the metal or alloy, the iinished casting will require but little attention in fitting it into the accurately, previously prepared, cavity.

In order to make the casting conform to a predetermined size and shape, that is, to the original pattern, it is important that the mold be expanded suiiiciently, just prior to the casting operation, to counteract the shrinkage in the metal or alloy that takes place when the metal cools and solidifies during this casting process. This shrinkage to be counteracted varies from .9% to over 2.%, depending on the size and shape of the casting and type of casting alloy used. For dental inlays, it varies from approximately .9% to 1.4%

The investments now commercially available, and termed simple investment compositions, consist principally of a binder, a refractory filler, and one or more modifying agents. The binder generally consists of plaster of Paris. 'I'here are several forms of calcium sulfate that may be used and which will act similar to plaster of Paris. Plaster of Paris is known as hemihydrate (Casole/2me) and generally has a testing consistency of 60 parts of water to 100 parts of plaster. Another form of hemihydrate known as alpha gypsum and commercially known as Hydrocal generally has a testing consistency of approximately 40 parts of water to parts of alpha gypsum. This lower consistency is advantageous in increasing the expansion and also the strength of the mold. There are several other forms of calcium sulfate, both hydrated and dehydrated, that set when gaged with water and may also act as a binder. The term binder will be used hereinafter to cover any, or a mixture, of the above forms. All test results and curves wherein the term plaster is referred to hereinafter were made with alpha gypsum or Hydrocal, which, as mentioned above,

is the preferred form.

The refractory flller generally used consists principally of silicious matter. Since the chief property desired, and the most diiiicult of attainment, in an investment composition is a high thermal expansion, it is obvious that refractory fillers with inherent high expansions are to be preferred. Silica, in its three principal modications, quartz, tridymite and cristobalite, possess high expansions along with good refractory properties and in addition are fairly inexpensive. The quartz form is the least expensive and almost always used. Hereinafter, the term silica may be construed as meaning quartz. Of the three forms, cristobalite has the greatest expansion and when used to replace all or part of the quartz form, will impart a greater expansion to the mold in proportion to the amount used.

Many other types of refractory materials may be used to replace all or part of the silica andinclude such refractory materials as magnesia, alumina, chromium oxide, refractory clays and many silicates. A number of silicates, and in fact several oxides, alone have unusually high expansions due to change of phase or inversion. Wherever these materials have melting points suniciently high to permit successful use in investments they may be used advantageously as part or all of the refractory ller.

The term refractory filler when used hereininafter shall be construed to include any of the above mentioned refractories, alone or in any mixture thereof, or any material having substantially the same property of withstanding the temperatures encountered in the casting process and which are compatible with the other ingredients.

In addition to the binder and refractory filler, it is desirable to add one or more of the following agents to adjust the setting or hardening characteristics, improve the smoothness or porosity of the mold, or otherwise impart desirable physical or chemical action to the mold surface so as to produce the smoothest, densest, and least contaminated Acasting possible. Such materials include accelerators, retarders, reducing agents (such as graphite), lne clay, powdered metals 'materials include boric acid, cristobalite and chlorides of sodium, potassium and lithium. Each of these has a particular value in increasing the expansion within limited temperature ranges, but generally effect a shrinkage in the molds at the higher temperatures, especially 1600-1800" F.

It is therefore the principal object of this invention to provide an investment composition for use in casting, principally, by the disappearing wax pattern method that will withstand high temperatures of heat and give the desired or necessary amount of thermal expansion at these high temperatures.

It is also an object of this invention to provide an investment composition for making molds that is considerably stronger than those heretofore used at temperatures where at least equal expansion is obtained.

It is a further object of this invention to provide an investment composition having a greater thermal expansion than those previously known eand used Without employing a greater amount of the thermal expansion improving agent.

It is a still further object of this invention to provide an investment composition in which the setting expansion and setting time can be controlled or modiied by the thermal expansion improving agent itself.

Other objects and advantages of this invention can be readily apparent by reference to the following specification, considered in conjunction with the accompanying drawing forming a part thereof, and it is understood that any modifications may be made within the scope of the appended claims, without departing from or exceeding the spirit of the invention.

l In the drawing:

Fig. 1 shows a pair of graphs illustrating the expansion of two investment compositions when heated under identical conditions.

Fig. 2 shows a pair of graphs of investment compositions illustrating the expansions when heated to a single given temperature with different amounts of the thermal expanding improving agent, each with the saine base respectively.

As Was noted above, general or simple investment compositions for dental and other purposes have been known and used for many years. This general compositionhas comprised from 18 to 50% plaster and 82 to 50% silica. The thermal expansions of such compositions increases with an increase in the silica content, but the strength of the resulting mold correspondingly decreases. 'I'he proportions of these ingredients as just set forth may foe varied beyond these limits, depending upon the exact technique used in .producing castings.

In Fig. 1 of the drawing, the reference numeral l0 indicates the thermal expansion (and contraction) of a mixture of 44% plaster and 56% silica, andindicates a thermal expansion of .64% between room temperature and 1300 F. 'I'his investment was made with a water to powder ratio (W/P) of .26, that is 26 parts of water to 100 parts of the powder mixture on a weight basis. An increase of the W/P causes -a material decrease in the thermal expansion of this type of mixture. It should be noted, however, that while the converse is true, the W/P above set forth is from a commercial standpoint about as low as is practicable.

In Fig. 1 of the drawing, the curve indicated by the reference numeral il shows the thermal expansion resulting from a modiiication of the simple investment composition by adding to said simple investment composition one part by weight of potassium icdide for an equal amount of silica. In this composition, the W/P ratio was held as above, that is, .26. As will be seen, this resulted in a thermal expansion of 1.25% between room temperature and 1300 F.

By comparison of the curves I0 and Il, it will y be noted that the 1% of potassium iodide produced an investment composition that was throughout the heating period expanding regularly, while the other or simple investment composition expanded, contracted and then reexpanded; In commercial practice, especially on comparatively large molds, there is often a temperature diierential of several hundred degrees between diiferent parts of the mold. Obviously, if one part is expanding and another contracting, distortion and cracking often takes place. In the modified composition, such danger is practically eliminated. It should be noted at this point that gold alloys are generally cast around 1300 F., and at this casting temperature have a casting shrinkage of 1.25%. The potassium iodide composition at this casting temperature has exactly this thermal expansion, namely, 1.25%, whereas the simple investment composition has practically one half of the necessary thermal expansion.

As was noted above, other thermal expanding improving agents had been employed, such for example as sodium chloride and boric acid. It should .be noted that one percent of sodium chloride in place of the one percent potassium iodide would give only 1.00% expansion, while one percent of boric acid Iwould only give approximately .9% thermal expansion.

In Fig. 2, the curve indicated by the reference character i 2 shows the thermal expansion of a simple composition when augmented with different amounts of the thermal expanding improving agent potassium iodide, as mentioned above. In other Words, 2% or parts of potassium iodide for an equal amount of silica lncreases the thermal expansion at 1300 F. to 1.36%; 3% or parts of potassium iodide for a similar amount of silica raised the thermal expansion to 1.41%, while 4% or parts of potassium iodide raised the thermal expansion to 1.45%. From the foregoing, it will be noted that the more potassium iodide employed, the greater the thermal expansion at a given temperature. It will also be noted that the amount of increased thermal expansion does not proportionately increase in accordance with the increased amount of potassium iodide used, and that a limit will be reached beyond which no further increase in thermal expansion will take place. This maximum amount being approximately 5 to 8% or parts of the total solid ingredients of the in-A vestment. 'Ihe greatest thermal expansion increase being obtained with the rst unit of potassium iodide added to the composition and that mal expansion.. From the foregoing, il; will also be noted that should less thermal expansion be desired, such as is obtained with sodium chloride silica content in the simple investment composition (with a consequent decrease in plaster) vthe thermal expansion can be increased, and

this thermal expansion can be further increased by the`substitution of small amounts of potassium iodide for corresponding amounts ofthe silica. The curve in Fig. 2, indicated by the reference character I3, is an example of such a composition, namely, a composition which includes plaster. 0 to 40% potassium iodide and the balance silica. By comparing curves II and I2, it will be noted that the decrease in plaster (increase in silica) has resulted in a curve having approximately .1% more thermal expansion throughout the range-illustrated. It should be further noted that these curves, indicated by reference characters I2 and I3 are summary plots of the 1300 points on the thermal expansion curves in which varying amounts of potassium iodide were added to simple investment base compositions. The small letter :c found in Figs. 1 and 2, and on curves denoted by reference numerals II and I2, and connected with a dash line, are the same points. It may also be noted that if the summary plots were made for any other temperature, the shape of the curves in Fig. 2 would be the same, except that the vertical intercepts would be raised or lowered in the same amount shown on the curve denoted by reference numeral II in Fig. 1. For example, if the temperature chosen is 1500 F., the Whole curves in Fig. 2 would be raised by the amount shown by the vertical distance between points :c and 'y in curve I I in Fig. 1, namely, .1%. Likewise, a summary plot at a lower temperature would correspond, but at a lower point in vertical heighth.

Whereas, in the above simple investment composition the increase in W/P causes a decrease in the expansion, the same is true with compositions containing the addition of potassium iodide, but not to such a marked degree. Obviously, this is advantageous where care is not taken to adjust the W/P ratio accurately. It is also to be noted that since the decrease in W/P increases the strength of the mold, the addition of any material which will not affect the thermal expansion or other casting characteristics of the investment composition and still enable a further decrease in W/P or impart to the composition a greater plasticity at the same W/P will be of added advantage. Some of the present sulfated higher alcohols or similar acting compounds, offer a slight advantage along this line. y

The foregoing description deals specically with potassium iodide as the thermal expansion improving agent; it has been found however that substantially all the other soluble inorganic iodides have a similar thermal expansion improving eifect. Potassium iodide being a widely used commercial chemical is the least expensive and least deliquescent of the iodides, and thereit is advisable to produce a mold wherein the cation potassium may not be compatible with the other ingredients, or where some of the other iodides perhaps available commercially in a more impure for-m and consequently less costly or offers other advantages, substantially equal increase in thermal expansion will be produced by the following iodides, namely, those of ammonium, potassium, sodium, lithium, rubidium, caesium, barium, calcium, strantium and magnesium. The thermal expansion is increased to a somewhat lesser degree by the iodides of the following cations or metals, namely, iron, zinc, nickel, cobalt, chromium, manganese and aluminum. Somewhat less desirable, but still able to producev considerable increase in expansion are the following iodides, namely, cadmiumpotassium, cadmium, copper, lead, bismuth, antimony and tin. Hydrogen iodide or hydriodic 'acid will also produce considerable benet in expansion increase. Should this acid be used, gas bubbles will usually be formed by reaction with impurities in the plaster, but may be removed by vigorous mixing. Other soluble inorganic iodides were not commercially available at the time tests were made, however, it would be fairly safe to predict that their action would be similar to those mentioned above.

While the iodides are generally decomposed at high temperatures, this does not appreciably affect the expansion properties of the composition. For example, ammonium iodide is more than likely entirely eliminated at the usual casting temperatures but nevertheless it has imparted some action, at present not explainable scientifically, to the mold which enables expansions to continue even though the added agent is no longer present.l The iodides are also generally benecial as cleansing agents in improving the surface appearance of the casting. Some of the iodides may form oxides when heated but nevertheless their beneficial expansion increasing and cleansing properties are not imparted to the mold if the respective oxides were originally added in place of the iodide.

From most standpoints, including beneficial effects on setting and thermal expansions, as well as general action as accelerators, the above iodides act substantially the same as the potassium iodide which was used throughout as an example. It is to be noted, however, that while the amounts used may have some lslight benefit above 5%, the cost involved and the fact that they are somewhat less desirable from a moisture absorbing standpoint, it would be desirable to use perhaps not over 3% of these.

fore the most suitable for these reasons. Where In general commercial practice, the various iodides would be added and mixed with the plaster and silica powders, but where thel moisture absorbing characteristics are undesirable, the same benecial results may be secured by adding the iodide to the gaging water instead.

As mentioned above, when describing the refractory filler in an investmenty composition, materials other than quartz may be substituted for all or part of the latter. APerhaps the most suitable refractory now available which will impart greater expansion to the mold than -its equivalent amount of quartz is cristobalite. Using 44% plaster with 1% potassium iodide and 55% silica, the thermal expansion at 1300 F. will be 1.25%.' By substituting cristobalite for all of the quartz in the above investment composition, Vthe thermal expansion will be raised to approximately 1.7 to 2%. depending upon the type and purity of4 the cristobalite. AWhere only a portionof thequartz is replaced by an equal `amount of cristobalite, the expansion will be increased in proportion to the amount of cristobalite used.' Thus, using 44% plaster and 1% potassium iodide, and with varying amounts of quartz and cristobalite any thermal expansion between 1.25% and approximately 1.7 to 2.0% nay be produced. Likewise, increase in the amount of iodide will increase the expansion of any given base containing cristobiatlite.A

A further advantage in the use of the iodides as a thermal expansion improving agent is the fact that molds containing same may be heated to temperatures of approximately 1 600-1800 F. without experiencing unduly rapid shrinkage as is the case with many of the other expansion improving agents and as is also true of the investment composition base itself. This is especially desirable inasmuch as alloys now used in dentistry made of base metals may be successfully cast in a mold that has suiiicient expansion within the casting temperate range demanded by these higher fusing alloys and which temperature range often approaches as high as 1600 to 1800 F.

What is claimed is:

1. An investment composition for casting metals and their alloys, consisting principally of a. refractory ller and a binder in such proportions as to give an investment composition and containing a soluble inorganic iodide, with such,

iodide being present in a quantity sufficient to improve the expansion properties of the composition, but not in excess of 4.5%.

2. An investment composition for casting metals and their alloys, containing from 18 to 50% binder, from 82 to 50% cristobalite, and a soluble inorganic iodide, with such iodide being present in a quantity sufficient to improve the expansion property of the composition, but not in excess of 4.5%.

3. An investment composition for casting met.- als and their alloys, consisting principally of a refractory filler, some of which' is in the form oi cristobalite, and a binder in such proportion as to give an investment composition and containing a soluble inorganic iodide in such proportion to improve the expansion property of the composition,but not in excess of 4.5%.

4. An investment composition for` casting metals and their alloys, consisting principally of crisobalite and quartz, and a binder in such proportion as to give an investment composition and containing a .soluble inorganic iodide in such proportion as to improve the expansion property of the composition, but not in excess of 4.5%.

5. An investment composition for casting metals and their alloys, consisting principally of a refractory filler and a binder in such proportion as to give an investment composition, and containing an iodide selected from the group containing ammonium, potassium, sodium, lithium, rubidium, caesium, barium, calcium, strontium, magnesium, iron, zinc, nickel, cobalt, chromium,

manganese, aluminum, cadmium-potassium. cadmium, copper, lead, bismuth, antimony, tin and hydrogen but not in excess of 4.5%.

6. An investment composition for castingv metals and their alloys, consisting principally of a. refractory ller and a binder, in such proportion as to give an investment composition, and containing an iodide selected from the group containing ammonium, potassium, sodium, lithium, rubidium, caesium, barium, calcium, strontium, magnesium, iron, zinc, nickel, cobalt, chromium, manganese, aluminum, cadmium-potassium, cadmium, copper, lead, bismuth, antimony, tin and hydrogen, in such proportion as to improve the expansion property of the composition but not in excess of 4.5%.

7. An investment composition for cast-ing metals and their alloys, 'consisting principally of a refractory nller and a binder in such proportion as to give an investment composition, and containing potassium iodide, With/ such iodide being present in a quantity suflici "nt to improve the expansion property of the yomposition, but not in excess of 4.5.

8. An investment composition for casting metals and their alloys, consisting principally of a refractory ller, and a. binder in such proportion as to give an investment composition, and containing ammonium iodide but not in excess of 4.5%.

9. An investment composition for casting metals and their alloys, consisting principally of a refractory filler and a binder in such propel tions as to give an investment composition, and containing ammonium iodide, wlth such iodide being present in a quantity suflicient to improve the expansion property of the composition, but not in excess of 4.5%.

10. An investment composition for casting metals and their alloys, containing from 18 to binder, from 82 to 50% cristobalite, and ammonium iodide, with such iodide being present in a quantity sufficient to improve the expansion property of the composition, but not in excess of 4.5%.

11. An investment composition for casting metals and their alloys, consisting principally oi a refractory iiiler, some of which is in the form of cristobalite, a binder in such proportion as to give an investment composition, and containing ammonium iodide in such proportion as to improve the expansion property of the composition, but not in excess of 4.5%.

12. An investment composition for casting metals and their alloys, consisting principally of a refractory filler and a binder in such proportions as to give an investment composition, and

containing sodium iodide, with such iodide being present in a quantity sufficient to improve the expansion property of the composition, but not in excess 0f 4.5%.

RBERT NEIMAN. ROBERT C. ERNST. EDMUND A. STEINBOCK. 

